Numerous configurations exist for an LED lens with a central refractor and an annular internal reflector. A parabolic reflector by itself has existed for more than a century. There are numerous patents directed to combinations of an internal reflector and a refractor. For example, in U.S. Pat. No. 1,977,689, Muller discloses a signal lamp for railroads employing a central refracting lens and an annular internal parabolic reflector as an improvement over metallic parabolic reflectors. The internal reflection of the annular collector is impervious to corrosion. Under the teachings of U.S. Pat. No. 2,215,900 to Bitner, the addition of several annular refractors greatly reduces the diameter of the parabolic reflector. Still further, an LED mounted at the focal plane of a concave mirror is claimed as a novel combination in U.S. Pat. No. 6,641,287 to Suehiro. An annular parabolic cap as taught in U.S. Pat. No. 4,698,730 to Sakai et al. collimates exiting light from the cylindrical surface of the LED lens. In U.S. Pat. No. 5,757,557 to Medvedev et al., a cylindrical void defines a collection angle beyond a hemisphere within an annular reflector and a central refractor. Still further, under the teachings of U.S. Pat. No. 6,547,423 to Marshall et al., a cylindrical void contains a convex refractor while the central refractor and the annular reflector share a planar exit face. There are numerous combinations of parabolic reflectors and refractors. Open windows for cooling are taught in U.S. Pat. No. 7,474,474 to Angelini et al.
A homogenizer distributes rays evenly throughout both space and angle. An integrating sphere may combine multiple inputs into a single output with excellent homogeneity throughout wavelength, space, and angle. A homogenizer may also be a hollow lightpipe, or a solid lightpipe. Diffuse reflectance greatly improves the distribution of rays throughout space and angle.
As shown below, a hemispherical emission is transformed by Snell's law of refraction into a much smaller angle within a refractive medium.sin θn=1/n Consequently, the angle of internal collection θn is much easier to collimate than a 90° angle within air.
The hemispherical collection efficiency can be determined as set forth below:
  HCE  =      2    ⁢                  ⁢          sin      2        ⁢          θ      2      
It can thus be appreciated that there is much more power per angle at higher angles. A central portion of the collection may be rejected in favor of other attributes as in U.S. Pat. No. 7,262,859 to Larson. The angle of collection is defined by the margin of lens. The marginal ray travels through the margin of the lens.
The optical power of transmission is much different than the optical power of internal reflection. At an air-to-glass interface (n=1.0, 1.5), the optical power of transmission can be determined as follows:
      ϕ    T    =      0.5    ⁢                  (                  1          R                )            .      The optical power of an internal reflection is determined by the equation:
      ϕ    IR    =            3      ⁢              (                  1          R                )              =          6      ⁢                          ⁢                        ϕ          T                .            
Thus, a radius for internal reflection has 6 times the optical power a radius for transmission. An internal reflector provides much more optical power per radius than a radius in transmission. Internal reflection employs a much longer radius than refraction at the same optical power.
As the brightness of an LED increases, the power supply becomes physically larger. A larger surface area is required for dissipation of heat. The electrical power source can be larger than the LED optics. This provides an opportunity for a larger optic.
A retroreflector normally comprises three orthogonal surfaces. A hollow reflector relies upon the large refractive index of a metal. A solid retroreflector relies upon total internal reflection. A retroreflector may also have just two reflecting surfaces.
The critical angle defines the internal angle at which the external angle is 90°. For example, the critical angle within a glass of index 1.5 in contact with air is 41.8°. At this internal angle of collection, the external angle collection is a hemisphere.
In an illustrative example, a fresh snow pack can nearly double the exposure of a person to UV light. This effect is due to numerous water-to-air facets with reflectance of approximately 2%. The diffuse reflectance of a non-absorbing dielectric approaches 100% as concentration of facets increases and the depth increases.
Thermoplastic resin reflectance material, such as that sold under the registered trademark SPECTRALON by Labsphere, Inc. of North Sutton, N.H. can be machined into a wide variety of shapes for the fabrication of optical components. The reflectance material gives the highest diffuse reflectance of any known material or coating over the UV-VIS-NIR region of the spectrum. The reflectance is generally >99% over a range from 400 nm to 1500 nm and >95% from 250 nm to 2500 nm and is resistant to UV degradation with NIST traceable calibration.
It will further be noted that the human eye can withstand only 5 mW of coherent radiation within the pupil. However, some LEDs exceed this limit. Therefore, LED illumination must be sufficiently spread over space to avoid discomfort or even damage to the human eye. A larger exit aperture can address the comfort of human vision.